Two-dimensional (2D) electrical imaging surveys are now widely used to map areas of moderately complex geology where conventional 1D resistivity sounding and profiling techniques are inadequate. The results from such surveys are usually plotted in the form of a pseudosection (Figure 1a) which gives an approximate but distorted picture of the subsurface geology.

The RES2DINVx32/x64 programs use the smoothness-constrained Gauss-Newton least-squares method inversion technique to produce a 2D model of the subsurface from the apparent resistivity data alone. It is completely automatic and the user does not even have to supply a starting model. This program has been optimised for the inversion of large data sets. The use of available memory is optimised so as to reduce the computer time by minimising disk swapping. On a modern microcomputer, the inversion of a single pseudosection is usually completed within seconds to minutes. Four different techniques for topographic modelling are available in this program. Together with the free 2D forward modeling program RES2DMOD, it forms a complete 2D resistivity forward modeling and inversion package.

The program will automatically choose the optimum inversion parameters for a particular data set. However, the parameters which affects the inversion process can be modified by the user. The smoothing filter can be adjusted to emphasize resistivity variations in the vertical or horizontal directions. Two different variations of the smoothness constrained least-squares method are provided; one optimised for areas where the subsurface resistivity varies in a smooth manner (such as chemical plumes), and another optimised for areas with sharp boundaries (such as massive ore bodies). A robust data inversion option is also available to reduce the effect of noisy data points. Resistivity information from borehole and other sources can also be included to constrain the inversion process. The complex resistivity method (Kenma, A., Binley, A., Ramirez, A. and Daily, W., 2000. Complex resistivity tomography for environmental applications. Chemical Engineering Journal, 77, 11-18.) is used for IP data inversion.

The figure below shows an example from an electrical imaging survey in an area with fairly complex subsurface geology and significant surface topography. This survey was carried out across a circular mound which is thought to contain some important Irish archaeological burial chambers (Waddell, J. and Barton, K, 1995, Seeing beneath Rathcroghan. Archaeology Ireland, Vol. 9, No. 1, 38-41.). The inversion of this data set, which has 67 electrode positions and 339 data points, takes seconds on a modern PC.

The second example is from a combined resistivity and IP survey over the Magusi River massive sulphide ore (Edwards L.S., 1977. A modified pseudosection for resistivity and induced-polarization. Geophysics, 42, 1020-1036.). This survey was conducted with 30.5 meters (100 feet), 61.0 meters (200 feet) and 91.4 meters (300 feet) dipoles. The resulting pseudosection has a complex distribution of the data points with overlapping data levels measured with different dipole spacings. The measured apparent resistivity and IP pseudosections, together with the model sections obtained are shown in Figure 2. The ore body shows up as a distinct low resistivity body with high IP values near the middle of the survey line in the model sections. Note the sharp boundaries between ore body and the surrounding rocks.

Now available as a combined package together with RES2DINVx32/x64, the 2D Resistivity & IP inversion program.
Supports exact and approximate least-squares optimisation methods
Supports smooth and sharp constrasts inversions
Supports up to 5041 electrodes and 67500 data points on computers with 1GB RAM (RES3DINVx32)
Supports more than 20000 electrodes and 100000 data points on computers with 16GB RAM (RES3DINVx64)
Supports trapezoidal survey grids
Supports surveys with electrodes at arbitrary positions (RES3DINVx64)
Supports parallel calculations on Intel (and compatible) based computers
Multi-core support with RES3DINVx32/x64, 128GB memory support with RES3DINVx64

In areas where the geological structures are approximately two-dimensional (2D), conventional 2D electrical imaging surveys have been successfully used. The main limitation of such surveys is probably the assumption of a 2D structure. In areas with complex structures, there is no substitute for a fully 3D survey. The arrays supported include the pole-pole, pole-dipole, inline dipole-dipole, equatorial dipole-dipole and Wenner-Schlumberger and non-conventional arrays.
The RES3DINVx32/x64 programs use the smoothness-constrained Gauss-Newton least-squares inversion technique to produce a 3D model of the subsurface from the apparent resistivity data alone. Like RES2DINV, it is completely automatic and the user does not even have to supply a starting model. A Intel multi-core (or compatible CPU) based microcomputer with at least 2 GB RAM and an 500 gigabyte hard-disk is recommended. It supports parallel calculations that significantly reduces the inversion time. On a modern microcomputer, the data inversion takes less than a minute for small surveys with 100 electrodes in a flat area, to several hours for extremely large surveys with 6000 electrodes in rugged terrain. Topographic effects can be modelled by using a distorted finite-element grid such that the surface of the grid matches the topography.
The program will automatically choose the optimum inversion parameters for a particular data set. However, the parameters which affects the inversion process can be modified by the user. On a modern multi-core Windows-based microcomputer, the data inversion takes from less than a minute for small surveys with 100 electrodes in a flat area to several hours for extremely large surveys with 6000 electrodes in rugged terrain. The inversion of a data set with 198 electrode positions (BLOCKS_22x9-ws.dat example data file) just takes about 17 seconds on a PC with a hex-core i7 CPU. Two different variations of the smoothness constrained least-squares method are provided; one optimised for areas where the subsurface resistivity varies in a smooth manner (as in many hydogeological problems), and another optimised for areas with sharp boundaries (such as massive ore bodies). A robust data inversion option is also available to reduce the effect of noisy data points.
To handle very large data sets, a data compression technique is used. It enables the inversion of very large data sets with over 50000 data points and model cells. As an example, RES3DINVx64 took about 3.6 hours to carry out 5 iterations for the inversion of a data set from an area with significant topography with 147,355 data points, 9396 electrode positions, 64,400 model cells and over 670,000 nodes on a PC with a 3.5GHz Hex-Core Intel 3930 CPU with 64GB RAM.
An example of the results obtained from an electrical imaging survey in an area with a very complex subsurface geology is shown in Figure 1. This survey was carried out at Lernacken in Southern Sweden over a closed sludge deposit using the pole-pole array (Dahlin, T. and Bernstone, C., 1997. A roll-along technique for 3D resistivity data acquisition with multi-electrode arrays, Procs. SAGEEP’97 , vol 2, 927-935.). A survey grid of 21 by 17 electrodes with a 5 metres spacing between adjacent electrodes was used. The former sludge ponds containing highly contaminated ground water show up as low resistivity zones in the top two layers. This was confirmed by chemical analysis of samples. The low resistivity areas in the bottom two layers are due to saline water from a nearby sea. Figure 2 shows a 3D plot of the inversion model using the Slicer/Dicer plotting program.

Figure 1. The 3D model obtained from the inversion of the Lernacken Sludge deposit survey data set. The model is shown in the form of horizontal slices through the earth.

Figure 2. The 3D model obtained from the inversion of the Lernacken Sludge deposit survey data set displayed with the Slicer/Dicer program. A vertical exaggeration factor of 2 is used in the display to highlight the sludge ponds. Note that the colour contour intervals are arranged in a logarithmic manner with respect to the resistivity.

Figure 3 shows an interesting example of a 3D resistivity and IP survey provided by Arctan Services Pty. and Golden Cross Resources, Australia. Copper Hill is the oldest copper mine in NSW, Australia. Copper porphyry with minor gold and palladium mineralization were found to occur in structurally controlled fractures and quartz veins. However, due to the very complex geology, large differences in ore grades were found in drill-holes that were less than 200 meters apart.

To map the ore deposit more accurately, a new 3D resistivity and IP survey using the pole-dipole array was used. The survey covered a large (1.6 x 1.1km) area using a series of 1.6 km lines with a spacing of 25m between adjacent electrodes. The entire survey took 10 days giving a total of over 7000 measurements (Denne, R., Collin, S., Brown, P., Hee, R. and White, R.M.S. 2001. A new survey design for 3D IP inversion modelling at Copper hill. ASEG 15th Geophysical Conference and Exhibition, August 2001, Brisbane). A copy of the paper describing the survey and results can be downloaded from the www.arctan.com.au web site. Other interesting information about the mineralization at the Copper Hill area is available at the Golden Cross Resources Ltd. www.reflections.com.au/GoldenCross/ web site.

The data was inverted with the RES3DINV program that produced a 3D resistivity as well as a 3D IP model for the area. The 3D IP model below shows the location of the mineralized zones more clearly. The inversion model output from the RES3DINV program was rearranged into a VRML format that could be read by a 3D visualization program (please contact Arctan Services for the details) that enables the user to display the model from any direction.

Figure 3. The IP model obtained from the inversion of the Copper Hill survey data set. Yellow areas have chargeability values of greater than 35 mV/V, while red areas have chargeability values of greater than 45 mV/V (White at al. 2001).

The above model shows two en-echelon north-south trends and two approximately east-west trends forming an annular zone of high chargeability. The results from existing drill-holes that had targeted the shallower part of the western zone agree well with the resistivity and IP model. A drill-hole, CHRC58, intersected a 217m zone with 1.7 g/t gold and 0.72% copper coincided well an IP zone of greater than 35mV/V. The lower boundary of the western zone with high chargeability coincides well with low assay results from existing drill-holes. The eastern zone with high chargeability and resistivity values do not outcrop on the surface and very little drilling has penetrated it. Further surveys, including drilling, is presently being carried out.